1. Field of the Invention
The present invention relates to a semiconductor laser structure, and more particularly, to a semiconductor laser structure in which a ridge waveguide and a grating are designed to form a predetermined angle to thereby obtain a pure single-wavelength laser.
2. Discussion of Related Art
Generally, a distributed feedback semiconductor laser structure is used to obtain a single-wavelength laser beam. The distributed feedback semiconductor laser structure includes an active layer, clad layers formed on and under the active layer, and gratings formed on and under the clad layers and having a pitch corresponding to an integer multiple of ½ a desired wavelength (nλ/2, n=1, 2, 3 . . . ). However, even when the distributed feedback semiconductor laser structure having the above-described structure is manufactured, a beam having two or more wavelengths is emitted. Therefore, it is not easy to obtain a pure single wavelength.
A semiconductor laser structure invented to overcome the above problems is described below. FIG. 1 schematically illustrates a conventional semiconductor laser structure having a grating structure. Referring to FIG. 1, the conventional semiconductor laser structure 100 includes a clad layer 110, an active layer 120 formed on the clad layer 110, upper and lower light guide layers 130 and 135 formed on and under the active layer 120, and a plurality of gratings 140 formed on the upper light guide layer 135. The plurality of gratings 140 have a period corresponding to an integer multiple of ½ a wavelength ((λn/2), n=1, 2, 3 . . . ), and include in their structure a region (ΔΦ) that varies by a length of one-quarter of a wavelength. ΔΦ may be represented by ((λ/2)n+λ/4, n=1, 2, 3 . . . ).
However, the semiconductor laser structure including the above-described grating structure disturbs a beam amplified in the direction of a resonant axis. Accordingly, when the above-described semiconductor structure is used, only a beam having a single wavelength remains, and a beam having a different wavelength is eliminated to thereby emit a laser beam having a pure single wavelength.
However, to produce a phase shift in a region of the grating structure as described above, since it is not easy to use a hologram lithography process using a general interference fringe, additional processes are required.
FIG. 2 schematically illustrates another conventional semiconductor laser structure having a grating structure. The semiconductor laser structure 200 of FIG. 2 includes an InP substrate 210, diffraction gratings 220 formed on the InP substrate 210, a first light guide 230 formed on the InP substrate 210, an active layer 240 formed on the first light guide 230, a second light guide 235 formed on the active layer 240, an InP clad layer 260 formed on the second light guide 235, an absorption type diffraction grating layer 270 formed in the InP clad layer 260, and an InGaAs layer 280 formed on the InP clad layer 260. The semiconductor laser structure 200 of FIG. 2 having the above-described structure is provided to overcome the above problems (the problems described with reference to FIG. 1) that it is difficult to use the hologram lithography process, which has a structure capable of using a conventional hologram method and producing a phase shift.
However, to form the above-described structure, a lithography process is additionally required in spite of using the conventional hologram method, and a precise process for adjusting the positions is required to respectively install diffraction gratings, i.e., the gratings on and under the active layer. In addition, when quantum dots are used in the active layer of the semiconductor laser structure, due to mismatch between the quantum dots, a multilayer structure and a long resonant axis are required so that the semiconductor laser obtains sufficient outputs, which results in multiple-wavelength oscillation.